INFLUENCE OF N-SUPPLY ON DEVELOPMENT AND DRY-MATTER ACCUMULATION OF AN OLD AND A NEW MAIZE HYBRID

The response of an old and a new maize (Zea mays L.) hybrid to N supply was evaluated under controlled-environment conditions. An old hybrid (Pride 5) and a new hybrid (Pioneer 3902) were grown at three N levels (15, 2.5, and 0.5 mM N), and development and dry matter accumulation were measured at the 4-, 8-, and 12-leaf stage. Leaf chlorophyll was monitored from the 8- to 12-leaf stage, and leaf photosynthesis, stomatal conductance, and leaf chlorophyll fluorescence were measured at the 12-leaf stage. Rate of dry matter accumulation decreased with declining N supply. fn contrast, decreasing N supply progressively increased the duration of the 4- to 8-leaf phase and the 8- to la-leaf phase. As a consequence, total dry matter at the 12-leaf stage did not differ among N levels. The rate of leaf appearance of the old hybrid (Pride 5) was proportionately more reduced under low N than that of the new hybrid (Pioneer 3902). Similarly, the reduction in crop growth rate during the 8- to 12-leaf stage at the lowest N level was 46% for Pride 5 and 24% for Pioneer 3902. Leaf chlorophyll content was higher for Pride 5 than for Pioneer 3902 at high N levels, but leaf chlorophyll content declined significantly more rapidly in Pride 5 than in Pioneer 3902 when N stress increased. Leaf carbon assimilation rates were 4-27% higher for the new hybrid, with the largest differences occurring at low N levels. Collectively, results of this study show that the old hybrid (Pride 5) was more sensitive to N stress during early development than the new hybrid (Pioneer 3902).